Bonded polyimide fuel cell package and method thereof

ABSTRACT

Described herein are processes for fabricating microfluidic fuel cell systems with embedded components in which micron-scale features are formed by bonding layers of DuPont Kapton™ polyimide laminate. A microfluidic fuel cell system fabricated using this process is also described.

The United States Government has rights in this invention pursuant toContract No. W-7405-ENG-48 between the United States Department ofEnergy and the University of California for the operation of LawrenceLivermore National Laboratory.

BACKGROUND

This invention relates to fuel cells. Work is commonly derived from fuelby a combustion process which uses the pressure of expanding gases toturn a turbine or move a reciprocating piston and, ultimately, toprovide torque to a driveshaft. This torque is then usually used forpropulsion or to generate electrical power. In the latter case, theelectrical power is often reconverted into mechanical work.

SUMMARY OF THE INVENTION

An aspect of the invention includes an apparatus comprising: a pluralityof polyimide layers wherein, at least one layer contains at least oneresistive heater, at least one layer contains at least one microfluidicchannel, at least one layer contains at least anode manifold having aflowing means for a fuel to flow to an anode, at least one layercontains a cathode manifold having removing means for removing reactionby-products from the cathode; an MEA layer, wherein the MEA comprises anelectrolyte sandwiched between an anode and a cathode; an electricalfeedthrough extending through all layers; and a fuel feedthroughextending through at least four of the layers to form a bonded polyimidemicrofluidic fuel cell system.

Another aspect of the invention includes an apparatus comprising: afirst polyimide layer containing a plurality of resistive heaters; asecond polyimide layer for insulating the resistive heaters; a thirdpolyimide layer containing a plurality of microfluidic channelscommunicating with a fuel source at one end and a fuel feedthrough atthe other end; a fourth polyimide layer comprising an anode manifoldcontaining (a) a fourth layer fuel inlet communicating with the fuelfeedthrough, (b) a porous membrane, (c) a fourth layer fuel outlet,wherein the fourth layer fuel inlet and outlet are configured such thatfuel flows horizontally through the fourth layer fuel inlet, thenvertically up through the porous membrane, wherein whatever portion offuel that does not flow vertically through the porous membrane continuesto flow horizontally through the fourth layer fuel outlet; a fifthpolyimide layer for supporting the anode manifold and containing a fifthlayer fuel feedthrough communicating with the fourth layer fuel outlet;an MEA layer containing an MEA, wherein the MEA comprises an electrolytesandwiched between an anode and a cathode, the MEA operating at atemperature less than 200° C., wherein the anode is positioned such thatit is communicating with the porous membrane of the fourth layer,wherein the fifth layer forms a seal between the porous membrane and theanode; a sixth polyimide layer for supporting a cathode manifold,wherein the sixth polyimide layer contains a sixth layer fuelfeedthrough communicating with the fifth layer fuel feedthrough; and aseventh polyimide layer containing (a) a cathode manifold comprising aplurality of microchannels communicating with the cathode, wherein themicrochannels are used for removing reaction by-products from thecathode, wherein the sixth layer forms a seal between the cathodemanifold and the cathode, and (b) a seventh layer fuel feedthroughcommunicating with the sixth layer fuel feedthrough; wherein anelectrical feedthrough extends through all the layers of the apparatusto form a bonded polyimide microfluidic fuel cell system.

A further aspect of the invention includes a process comprising:patterning a plurality of polyimide preform layers; bonding a portion ofthe plurality of polyimide preform layers together; positioning an MEAlayer containing an MEA onto the bonded portion of polyimide preformlayers; positioning the remainder of the perform layers onto the MEA;and curing to form a bonded polyimide microfluidic fuel cell system,wherein the bonded polyimide microfluidic fuel cell system has a meansto electrically connect all layers.

A further aspect of the invention includes a process comprising:patterning a first plurality of polyimide preform layers, wherein atleast one layer has means to heat an MEA, at least one layer has meansto distribute fuel to the anode of the MEA; bonding a the firstplurality of polyimide preform layers together; positioning an MEA layercontaining an MEA onto the bonded polyimide preform layers; patterning asecond plurality of polyimide perform layers, wherein at one layer hasmeans to remove bi-products from the cathode of the MEA; positioning thesecond plurality of perform layers onto the MEA; and curing to form abonded polyimide microfluidic fuel cell system, wherein the bondedpolyimide microfluidic fuel cell system has a means to electricallyconnect all layers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an expanded 3-dimensional layout of one embodiment of amicrofluidic fuel cell system.

FIG. 2 shows an expanded view of a bonded and sealed microfluidic fuelcell package formed using a polyimide molding process.

FIGS. 3A through 3L show the layers of one embodiment of a bondedpolyimide microfluidic fuel cell system.

DETAILED DESCRIPTION

Described herein is a process for fabricating microfluidic systemssuitable for use with a variety of fuel cells that operate attemperatures up to 200° C. These fuel cells are described in detail inpending U.S. patent application Ser. No. 09/241,159, now U.S. Pat. No.6,638,654 issued Oct. 28, 2003 “MEMS-based thin film fuel cells”,assigned to the same assignee, which is hereby incorporated byreference. The microfluidic systems described herein can be arranged inseries and/or in parallel and have power outputs ranging from 100milliwatts to 20 Watts.

The foundation of the fuel cell is formed by a polyimide laminatecomprising layers of DuPont Kapton™ (the trade name for polyimide). Eachlayer is coated with Dupont TEFLON® FEP fluorocarbon film that is bondedto Dupont Kapton™ HPP-ST polyimide film. An anode electrode is depositedon top of or attached to a porous membrane. The porous membrane can be alayer of polyimide into which a high density array of very narrowchannels, i.e., an array greater than 10⁴ channels/cm² having channelwidths or diameters less than 10 μm, has been cut. This array can becreated in various patterns by a variety of etch techniques whichinclude photolithography, wet or dry chemical etching, laser machiningor ion tracking lithography. A layer of polyimide having a thicknessranging from 25-50 μm is desirable regardless of which technique isused. The porous membrane can also comprise other plastic materialsalready having a high porosity, for example, those typically used inmicrofiltration. The plastic materials typically come in sheets and havesubmicron diameter pores and thicknesses ranging from 10-100 μm.Additional porous laminate layers can be used to embed a resistiveheater directly beneath the fuel cell device to control the fuel celltemperature.

These laminate bonded polyimide structures form the basis of themicrofluidic system that controls the delivery of gas or liquid fuels tothe membrane electrode assembly (MEA), also referred to as the fuelcell, portion of the fuel cell package. The MEA is theelectrode/electrolyte/electrode portion, including catalysts materialsand any additional components that may be sandwiched in between the twoelectrodes. For example, an effective MEA comprises a first Nafion™layer containing an anode catalyst, e.g., carbon or metal, and acatalyst material, e.g., Pt—Ru; a second Nafion™ layer as theelectrolyte; and a third Nafion™ layer containing a cathode catalyst,e.g., carbon or metal, and a catalyst material, e.g., Pt. These laminatebonded polyimide microfluidic structures can be used with a protonexchange membrane fuel cell, i.e., the Nafion™ MEA just described, orany fuel cell operating at temperatures up to 200° C. They can also beused to form a porous flow field structure for support of the MEA, formsuitable sealing and bonding of the structure, embed microfluidicconnection and control devices, including pumps and valves, andfacilitate scaling and stacking via continuous three dimensional flowchannels.

Micromachining is used to create a preform. The preform is then alignedand bonded to another layer of polyimide or another material such asPDMS, glass, or silicon. The surface comes prepared with a bondingadhesive, e.g., Dupont TEFLON® FEP, thereby forming a sealed bondsuitable for fluidic transport. Embedded within the sealed and bondedpolyimide laminate layers can be valves, fluidic connectors, siliconmicromachined components, and resistive heaters. In addition,high-density micropores can be formed in a thin layer of the polyimidelaminate material to create a porous membrane suitable for distributingfuel to a high surface area anode/cathode gas diffusion electrode fuelcell configuration.

FIG. 1 shows an expanded 3-dimensional layout of one embodiment of amicrofluidic fuel cell package formed using the polyimide laminatepreform bonding process. Referring to the embodiment shown in FIG. 1, anMEA 102 comprises an anode 104, a restive heater or a plurality ofresistive heating elements 106, a heater isolation layer 108, anelectrolyte 109 and a cathode 110. A micromachined manifold system 112and a support structure 114, which together are referred to as amicrofluidic fuel distribution structure 116, are bonded to MEA 102forming a sealed microfluidic fuel cell package 118. Fuel is able toflow horizontally through the manifold system and then vertically upthrough a porous membrane to the MEA.

The polyimide laminate bonding process can be utilized in the design offuel cells. A porous membrane is desired in the fuel cells that allowsthe fuel to flow through a microfluidic channel horizontally from acommon reservoir, then vertically through a prescribed array of holeswhich distribute the fuel over the surface area of the anode. The fuelflow may be assisted and controlled with micropumps and valves,including turnoff and isolation if necessary. The polyimide laminatemicrofluidic structure provides structural support for the fuel cellmembrane and creates a tight seal around the periphery of the MEA.

FIG. 2 depicts another embodiment of a bonded and sealed microfluidicfuel cell package 218 formed using a polyimide laminate preform bondingprocess. Micropumps and valves (not shown) as well as isolation or turnoff valves (not shown) can be incorporated into the fuel cell package218 to enable control of the fuel flow. The polyimide preform laminatecan form the microfluidic fuel distribution structure 216 which containsmicrofluidic interconnect ferrules 217 for fuel inlet and by productoutlet. An MEA 202 comprising an anode 204, an electrolyte layer 209 anda cathode 210, can be positioned directly on the preformed hoststructure 230, such as, microfluidic fuel distribution structure 216.This allows connection to a plurality of microfluidic fuel distributionchannels 222 through a plurality of micropores 223 covering the surfacearea of the anode, while allowing electrical connections, e.g., 226 a,to the anode to be fed through the seal. The polyimide preform laminatematerial can directly seal to a glass, silicon, ceramic, plastic, PDMSor other laminate material substrates by surface bonding.

Metalization to form resistive heating elements for direct control ofthe MEA temperature and feedthrough electrical connections to extractthe generated electrical power from the fuel cell can also beincorporated into the fabrication process. The process of metalizationonto the surface of the polyimide layers does require some manipulationof the surface. Plasma etching the surface in oxygen plasma at 300 Wattsfor 1 minute or sputter bias etching prior to deposition are botheffective surface treatments. An evaporation can be used to apply themetal to the surface of the plasma-etched substrate. An adhesion layer,such as chromium, is applied first at 2 angstroms/second for a200-Angstrom layer. A metal layer, such as gold, is then evaporated ontothe adhesion layer at 2 angstroms/second to obtain a 2000-Angstromlayer. Different patterns can be formed by using a shadow mask to blockthe evaporation over parts of the surface. A wide variety of metals canbe used for conductive wires embedded as feedthroughs to extract theelectrical power from the fuel cell electrodes. The choice of metaldepends on adhesion and conductivity. Typical metals used include Au,Ni, Cu, Ti, Ag, Pt, and Al. Once the metal has been applied to thesurface of the substrate, standard photolithography techniques can alsobe used to pattern the metal.

Other items can also be embedded within the polyimide laminate. Forexample, embedded capillary tubing can provide a leak-proof fluidicconnection between the reservoir and the fuel cell package without usingthe normally necessary connectors that screw into the fuel cell packageproviding a compression seal with the tubing. Devices typicallyfabricated in glass or silicon could be embedded into the polyimidelaminate making fluidic connections with capillary tubing even easierwithout the requirement of other hardware. This can be accomplished bythermoforming the polyimide film into a connector shape to whichcapillary tubing can be attached. Embedding the capillary tubing in thismanner allows the delivery of a gaseous or liquid fuel to a microfluidicfuel cell and the distribution of air to a gas diffusion cathodestructure.

Metal deposition techniques such as, sputter, e-beam, or screenprinting, can be used to form embedded structures, such as, a pluralityof resistive heating elements 206 for direct control of the MEAtemperature, and/or a plurality of feedthrough electrical connections226 a and 226 b to extract the electrical power generated. Electricalconnection 226 b can be sandwiched between the cathode and an airbreathing electrode support structure 227. Patterns may be formed in themetal layer using standard photolithography techniques once a layer ofmetal has been applied to the surface of the substrate.

Referring to FIG. 2, fuel can be distributed to the surface area of fuelcell anode 204 through a prescribed array of holes 223 in a microfluidicchannel 222 from a reservoir (not shown), with optional assistanceand/or control from micropumps and valves (not shown). Anode 204preferably has a large surface area, i.e, >10⁶/cm². At anode 204, thestructure is heated in the range of 60-90° C. to generate theelectrochemical and catalytic reactions, e.g., H₂→2H⁺+2e⁻ at the anode,and 4H⁺+2O⁻²→2H₂O+4 e³¹ at the cathode, which convert the fuel toelectrical power. With appropriate anode catalyst materials, such asPt—Ru, a vaporized feed of methanol-water mixture can be used.Typically, ratios of 0-50% Ruthium/50-100% Platinum are used. In thiscase, the anode reaction first converts the methanol-water mixturethrough the reaction CH₃OH+H₂O→3 H₂+CO₂, which then leads to the anodeand cathode reactions as described above.

Another embodiment of a bonded polyimide microfluidic fuel cell systemis shown in FIGS. 3A through 3L. Referring to FIG. 3A, a bondedpolyimide microfluidic fuel cell system 300 comprising a number ofbonded layers: a resistive heater polyimide layer 302, a firstinsulating polyimide layer 304, a microfluidic channel polyimide layer306, an anode manifold polyimide layer 308, an optional porous membranepolyimide layer 310, an anode pad polyimide layer 312, second insulatingpolyimide layer 314, an MBA layer 316, a metal pad polyimide layer 318,cathode manifold polyimide layer 320, and a cathode pad polyimide layer322.

Referring to FIG. 3B, resistive heater layer 302 comprises a pluralityof resistive heaters 324. The heaters can be formed by patterned shadowmasking and metal deposition techniques. The heater circuits 326terminate at conductive pads 328 a and 328 b. Resistive heater layer 302can further comprise an inlet 330 and an outlet 332, depicted withdotted lines.

Referring to FIG. 3C, first insulation layer 304 comprises electricalvias 334 a and 334 b to permit an electrical path to power the resistiveheaters of resistive heater layer 302. Electrical vias 334 a and 334 bare present in each succeeding layer of system 300 and are filled withan electrically conductive material, such as silver epoxy, once thesystem is assembled. First insulating layer 304 can further compriseinlet 330 and outlet 332.

Referring to FIG. 3D, microfluidic layer 306 comprises inlet 330, outlet332, electrical vias 334 a and 334 b, and at least one microfluidicinlet channel 336 that communicates with inlet 330 and at least onemicrofluidic outlet channel 338 that communicates with outlet 332.

Referring to FIG. 3E, anode manifold polyimide layer 308 comprises inlet330, outlet 332, electrical vias 334 a and 334 b, and anode manifold 340having at least on anode area 341. Anode areas 341 comprise slits in thepolyimide material of polyimide layer 308. Once assembled anode manifoldlayer 308 is positioned such that one end of the anode manifold willoverlap with the microfluidic channels of polyimide layer 306 and theother end of the anode manifold with overlap with the microfluidicoutlet channels of polyimide layer 306 providing a continuous path forfuel to flow. Fuel flows horizontally through inlet 330 and verticallyup through the anode manifold 340 such that whatever portion of the fuelthat is not consumed continues to flow horizontally through fuel outlet332.

Referring to FIG. 3F, an optional porous membrane polyimide layer 310 iseither a porous sheet of polyimide or a sheet of polyimide having porousareas 339 in line with the anode manifold of polyimide layer 308 suchthat polyimide layer 310 serves as an extension of anode manifold 340.Polyimide layer 310 further comprises inlet 330, outlet 332, electricalvias 334 a and 334 b.

Referring to FIG. 3G, anode pad polyimide layer 310 comprises at leastone anode pad 342 that terminates with a conductive pad 344. Additionalanode pads, if present, terminate with an anode contact pad 343. Metaldeposition and showdown masking is used to form the anode pads,conductive pad and contact pads.

Referring to FIG. 3H, second insulating polyimide layer 312 comprises aconductive pad via 346 that is positioned in the same location onpolyimide layer 312 as conductive pad 344 is on polyimide layer 310, ananode pad via 348 that is positioned in the same location on polyimidelayer 312 as anode pad 342, a contact pad via 350 positioned in the samelocation on polyimide layer 312 as anode contact pad 343 is on polyimidelayer 310, inlet 330, outlet 332, and electrical vias 334 a and 334 b.

Referring to FIG. 3I, polyimide layers 320, 304, 306, 308, 310 (ifpresent) and 312 are aligned one on top of the other and placed on astainless steel fixture 352, then sandwiched between two pieces of glass354 and 356, or other suitable material. A weight greater than 1 Kgwhich provides a force indicated by arrow 358 is placed on the sandwichand the entire fixture containing the polyimide layers is heated in anoven. The oven is ramped to 305° C. over 150 minutes (approximately 2°C./min.). Once the temperature reaches 305 ° C., the oven is turned offand the fixture is allowed to cool over several hours. Each polyimidelayer is coated with a bonding adhesive, i.e., Dupont TEFLON® FEP, toprior to alignment with the successive layer.

Referring to FIG. 3J, MEA layer 314 comprises a sheet of an electrolytematerial 360, with at least one anode 362 on one side and at least onecathode 364 on the other side, inlet 330, outlet 332, electrical vias334 a and 334 b, anode contact pad vias 350 and conductive pad via 346.Any MEA that operates at a temperature less than 200° C. is effective.Anode 362 is positioned such that it is communicating with anodemanifold 340 either directly or through porous membrane layer 310, ifpresent.

Referring to FIG. 3K, metal pad polyimide layer 316 comprises anelectrical contact metal pad 366 formed by a shadow mask, metaldeposition method, inlet 330, outlet 332, electrical vias 334 a and 334b, anode contact pad vias 350, anode vias 348 and conductive pad via346.

Referring to FIG. 3L, cathode manifold layer 318 comprises a cathodemanifold 368, one cathode conductive pad 370 terminating with aconductive pad 372, inlet 330, outlet 332, electrical vias 334 a and 334b. If more than one cathode pad is present, those cathode pads terminatewith a contact pad 374. The cathode pads, conductive pad, and contactpads can be formed by a shadow masking metal deposition technique.Cathode manifold 368 comprise slits in the polyimide material ofpolyimide layer 318. Once assembled cathode manifold layer 318 ispositioned such that reaction by-products can be removed from cathode364.

The bonded polyimide fuel cell system is completed by first coatingpolyimide layer 316 with a low temperature adhesive, i.e., DupontTEFLON® FEP, and positioning it on top of MEA layer 314 and secondcoating polyimide layer 318 with a low temperature adhesive, i.e.,Dupont TEFLON® FEP, and positioning it on top of polyimide layer 316.The surfaces of polyimide layer 316 and polyimide layer 318 form ahermetic seal when heated to 120° C. An electrical feedthrough,extending through all the layers is formed by filling electrical viaswith a conductive silver epoxy. Fuel cell system 300 is then cured at120° C. for 1 hour.

An illustrative example follows.

EXAMPLE 1

Patterning of the following preform layers using a laser etch technique;1.) Resistive heater-alignment targets, 2.) Heater isolation-alignmenttargets, electrical feedthroughs, 3.) anode manifold, microfluidicchannels, electrical feedthroughs, 4.) Anode support manifold, fuelinlet/outlet feedthroughs, electrical feedthroughs, 5.) anode manifoldseal, electrical feedthroughs, fuel inlet/outlet feedthroughs, 6.)cathode seals, fuel inlet/outlet seals, electrical feedthroughs, 7.)cathode support, cathode manifold, fuel inlet/outlet feedthroughs,electrical feedthroughs. Metal deposition is performed on layer 1 toform resistive heaters, layer 3 to form anode feedthroughs, and layer 6to form cathode feedthroughs. These are patterned by shadowmask.

The preformed layers are then aligned in order, first layers 1-5 areplaced on a fixture, sandwiched between two pieces of glass. A heavyweight is placed on them, and the entire fixture is placed in an oven,which is ramped to 305° C. over 150 min (approximately 2°/min). Once thetemperature reaches 305° C., the oven is turned off and allowed to coolover several hours. A membrane supported MEA is then sandwiched betweenlayer 5 and 6, creating adequate seals. The surfaces of layer 5 and 6have a low temperature adhesive which forms a hermetic seal by heatingto 120° C. Layer 7 is then aligned and bonded at 120° C. Electricalfeedthroughs are formed using a conductive silver epoxy which fills thevias and is cured at 120° C. for 1 hour.

All numbers expressing quantities of ingredients, constituents, reactionconditions, and so forth used in the specification and claims are to beunderstood as being modified in all instances by the term “about”.Notwithstanding that the numerical ranges and parameters setting forththe broad scope of the subject matter presented herein areapproximations, the numerical values set forth in the specific examplesare reported as precisely as possible. Any numerical value, however,inherently contain certain errors necessarily resulting from thestandard deviation found in their respective testing measurements.

While various materials, parameters, operational sequences, etc. havebeen described to exemplify and teach the principles of this invention,such are not intended to be limited. Modifications and changes maybecome apparent to those skilled in the art; and it is intended that theinvention be limited only by the scope of the appended claims.

1. An apparatus comprising: a plurality of polyimide layers wherein, atleast one layer contains at least one resistive heater, at least onelayer contains at least one microfluidic channel, at least one layercontains at least one anode manifold having a flowing means for a fuelto flow to an anode, wherein said flowing means comprise: (a) a fuelinlet, (b) a porous membrane, and (c) a fuel outlet, wherein said fuelinlet and said fuel outlet are configured such that said fuel flowshorizontally through said fuel inlet, then vertically through saidporous membrane to said anode, wherein whatever portion of fuel thatdoes not flow vertically continues to flow horizontally through saidfuel outlet, and wherein said porous membrane comprises a plasticmaterial having pores with diameters less than 1 μm and a thicknessranging from 10-100 μm, at least one layer contains a cathode manifoldhaving removing means for removing reaction by-products from a cathode;an MEA layer, wherein said MEA comprises an electrolyte sandwichedbetween said anode and said cathode; an electrical feedthrough extendingthrough all layers; and a fuel feedthrough extending through at leastfour of the layers to form a bonded polyimide microfluidic fuel cellsystem.
 2. The apparatus recited in claim 1, wherein (1) said layercontaining at least one microfluidic channel, (2) said layer containingat least one anode manifold, (3) said layer containing at least onecathode manifold and (4) said MEA layer communicate by a fuelfeedthrough.
 3. The apparatus recited in claim 1, further comprisingadditional layers comprising sheets of polyimide material.
 4. Theapparatus recited in claim 1, wherein said MEA operates at a temperatureless than 200° C.
 5. The apparatus recited in claim 3, wherein at leastone of said sheets of polyimide material are positioned between saidporous membrane and said anode forms a seal between said porous membraneand said anode.
 6. A apparatus recited in claim 1, wherein said removingmeans comprise: at least one of microchannel communicating with saidcathode, wherein an additional sheet of polyimide material forms a sealbetween said microchannel and said cathode.
 7. The apparatus recited inclaim 1, wherein said MEA comprises three perfluorosulfonic acid layers,said first perfluorosulfonic acid layer containing said anode comprisingan anode catalyst material, said third perfluorosulfonic acid layercontaining said cathode comprising a cathode catalyst material.
 8. Theapparatus recited in claim 1, wherein said anode and said cathodecomprise carbon cloth.
 9. The apparatus recited in claim 7, wherein saidcatalyst material comprises Platinim-Ruthenium in a ratio range of about50-100% Platinum to 0-50% Ruthinium.
 10. The apparatus recited in claim1, wherein said fuel is a methanol-water mixture.
 11. The apparatusrecited in claim 1, wherein said fuel is a methanol-water mixturecomprising hydrogen, carbon dioxide and less than 1% carbon monoxide.12. The apparatus recited in claim 1, wherein said porous membranecomprises a layer of polyimide having an array of greater than 10⁴channels per square centimeter, wherein the width of the channels isless than 10 μm.
 13. The apparatus recited in claim 1, furthercomprising two polyimide layer having at least one microfluidicconnection device embedded between the two layers, wherein saidmicrofluidic connection device communicates with said fuel feedthrough.14. The apparatus recited in claim 13, wherein said microfluidicconnection device is capillary tubing.
 15. The apparatus recited inclaim 1, further comprising at least two polyimide layers having atleast one pump embedded between the two layers, wherein said pumpcommunicates with said fuel feedthrough.
 16. The apparatus recited inclaim 1, further comprising at least two polyimide layers having atleast one valve embedded between the two layers, wherein said valvecommunicates with said fuel feedthrough.
 17. The apparatus recited inclaim 1, wherein the thickness of each polyimide layer ranges from25-200 μm.
 18. The apparatus recited in claim 1, wherein said apparatushas a power output ranging from 0.1 to 20 Watts.
 19. The apparatusrecited in claim 1, wherein said fuel feedthrough of said bondedpolyimide microfluidic fuel cell system communicates with a fuelfeedthrough of another bonded polyimide microfluidic fuel cell system,said bonded polyimide microfluidic fuel cell systems being connected inseries.
 20. The apparatus recited in claim 19, wherein said apparatushas a power output ranging from 100 milliWatts to 20 Watts.
 21. Theapparatus recited in claim 1, wherein said fuel feedthrough of saidbonded polyimide microfluidic fuel cell communicates with a fuelfeedthrough of another bonded polyimide microfluidic fuel cell system,said bonded polyimide microfluidic fuel cell systems being connected inparallel.
 22. The apparatus recited in claim 21, wherein said apparatushas a power output ranging from 100 milliWatts to 20 Watts.